Fun topic!
Balor's Best of Burzum's Ambient Material:
The Crying Orc (Burzum)
The One Who Wandered (Det Som Engang Var)
Emptiness (Hvis Lyset Tar Oss)
Decrepitude I (Filosofem)
Towards Ragnarok (Dauði Baldrs)
The Love of Nerthus (Hliðskjálf)
Decrepitude II (Filosofem)
While it does not represent the full breadth of his material, I think that it nevertheless encompasses some of the best of the early ambient works. I listed them in order of their release. Honestly, this would be one of my dream albums if it was ever compiled and released.